1. Field of the Invention
The present invention relates to a control device that is installed in a liquid crystal display apparatus and the like and that controls the light emission rate of the backlight thereof.
2. Description of the Related Arts
Like self-emission-type PDPs (plasma display panels), liquid crystal display apparatuses are being popularly used in television sets and various displays because they can provide a large display screen and have many other advantages, such as being lightweight, thin and of a low power consumption rate when compared with display apparatuses comprising a CRT (cathode ray tube). A liquid crystal display apparatus has a pair of transparent substrates of a size selected from a number of sizes conforming to standards, and liquid crystal is filled in the gap separating them from each other and held in a hermetically sealed condition. Then, the apparatus optically displays a given image as a voltage is applied to the transparent substrates to change the directions of the liquid crystal molecules and hence the transmittance of light of each of the molecules.
Since liquid crystal itself is not a light emitting body, a liquid crystal display apparatus is provided typically on the rear surface of the liquid crystal panel thereof with a backlight unit that operates as a light source. The backlight unit typically as a primary light source, a photoconductive plate, a reflector film, and a lens sheet or a diffusion film so that displaying light is supplied to the entire surface of the liquid crystal panel. In the past, a CCFL (cold cathode fluorescent lamp) containing mercury or xenon in a fluorescent tube in a hermetically-sealed condition was used as a primary light source. However, cold cathode fluorescent lamps have a number of problems to be resolved. They include a low luminance of emitted light, a short service life and a poor uniformity of light because of the existence of a low luminance region at the cathode side.
Large liquid crystal display apparatuses are generally provided with an area-lit configuration backlight unit that is formed by arranging a plurality of long cold cathode fluorescent lamps on the rear side of a diffusion plate in order to supply displaying light to the liquid crystal panel. However, such area-lit configuration backlight units are required to dissolve the above-identified problem attributable to cold cathode fluorescent lamps. Particularly, the requirement of high luminance and high uniformity is remarkable in the case of large television sets having a display screen greater than 40 inches.
LED area lit backlights formed by two-dimensionally arranging a large number of light emitting diodes (to be referred to as LED hereinafter) of three primary colors of light of red, green and blue on the rear surface of a diffusion film in place of the above described cold cathode fluorescent lamps are attracting attention. Such LED backlight units are being used in large liquid crystal panels to display images with an enhanced degree of luminance because they can be manufactured at low cost due to the falling cost of LEDs and operate with a low power consumption rate.
In various backlight units, various optical members including an optical function sheet block for changing the function of display light emitted from the light source, a diffusion/light guide plate, a light diffusion plate and a reflection sheet are arranged between the light source unit and the transmission-type liquid crystal panel. The light diffusion plate of the backlight unit is typically made of transparent acrylic resin and provided with a lighting control pattern that operates to partly transmit and partly reflect display light striking a site located opposite to the light source. Patent Document 1 [Japanese Patent Application Laid-Open Publication No. Hei 6-301034] describes a backlight unit comprising a light diffusion plate provided with a plurality of belt-shaped light control patterns arranged in a region located opposite to a fluorescent tube, each of the plurality of belt-shaped light control patterns being formed by a large number of reflection dots. On the light diffusion plate, the reflection dots are formed in such a way that the area of dots decreases as the distance of the dot from the axial line of the fluorescent tube increases. With this arrangement, the transmittance of light increases as the distance from the fluorescent tube increases so that consequently uniformized light is emitted as a whole.
Meanwhile, LEDs have the characteristics that the luminance degradation characteristics thereof change as the internal temperature rises and falls and LEDs of different colors have different sets of characteristics. Therefore, for the LEDs of each color to maintain the same and constant luminance level, it is necessary to detect the variation of luminance of the LEDs of each color by means of an optical sensor and correct the light emission rate of the LEDs of each color according to the detected value in a controlled manner.
Generally, the control device of an LED backlight unit is adapted to conduct a feedback control operation so as to make the LEDs of green color maintain the same and constant luminance level and the LEDs of red color and those of blue color are corrected for luminance in a controlled manner.
When correcting the LEDs for luminance, it is necessary to consider the change in the junction temperature θj that arises due to the temperature change in the LEDs of green color. If the range of change of the junction temperature θj is between 35° C. and 95° C., it is necessary to conduct a feedback control operation so as to increase the electric current being supplied to the LEDs of green color by more than 50%.
Additionally, the light emission efficiency of LEDs falls as the operation hours increase. For example, when an LED is driven to operate for light emission for about fifty thousands hours, while maintaining the junction temperature at a constant level of 90° C., its light emission efficiency falls to about 70° C. of the initial level. Therefore, when LEDs are used as a light source in a television set, it is necessary to raise the electric current supplied to the LEDs as a function of the total operation hours of the LEDs in order to maintain the luminance of the LEDs at a constant level. Still additionally, when the electric current is supplied to the LEDs of green light to a predetermined value (tolerance limit), it is no longer possible to correct the luminance thereof. Thus, the service life of the LEDs of green light comes to end when it is no longer possible to correct the luminance of the LEDs.
To avoid this problem, a control device 30 for controlling the light emission rate of LEDs typically comprises photo-sensors 31a, 31b for detecting the light emission rate of each of the LEDs, a temperature sensor 32 for detecting the quantity of heat of the backlight unit, a sensor input section 33 to be used for inputting the detected values of the photo-sensors 31a, 31b and the temperature sensor 32, a memory 34 storing the service life degradation curve and the temperature characteristic curve of LEDs of green light, a service life timer 35 for counting the total light emission hours of the LEDs, a SW/ON timer 36 for counting the elapsed light emission hours of the LEDs each time the power source is turned ON, a reference quantity of light outputting section 37 for outputting the quantity of emitted light that provides a reference for the LEDs of each color, a set conditions determining section 38 for determining the conditions of LED backlight to be set, an electric current supply section 39 for supplying an electric current to each LED according to the outcome of the determination of the set conditions determining section 38 and an operation section 40 for generating operation signals, and operates to control the backlight unit.
The set conditions determining section 38 performs a predetermined arithmetic operation on the basis of the detected values that are input to the sensor input section 33, the count value of the service life timer 35, the count value of the SW/ON timer 36 and the service life degradation curve and the temperature characteristic curve of LEDs of green light stored in the memory 34 and compares the computed value obtained as a result of the arithmetic operation and the reference value output from the reference quantity of light outputting section 37. Then, it determines the conditions of LED backlight to be set on the basis of the outcome of the comparison.
Thus, the control device 30 having the above-described configuration conducts feedback control operations for gradually lowering the target value for the luminance of each LED along the service life degradation curve stored in the memory 34 so that the value of the electric current supplied to each LED does not reach the tolerance limit and hence it is possible to maintain the balance of RGB for a long period of time.